434 Going Backwards Is Important!

The factors of 434 are listed below the puzzle, but here is a quick problem (really a puzzle) for you to figure out:

434 is the sum of six consecutive prime numbers. One of those primes is 71. What are the other five? If you would like, you may type your answer in the comments.

Solving that problem will require you to work backwards.

Many students may think they understand a mathematical concept, but they don’t really understand it unless they can also do it backwards. For example, they may understand, but maybe not remember that 7 x 8 = 56. Yet, these same students are completely baffled if you tell them this rhyme, “Five, six, seven, eight, 56 is 7 times 8.” They aren’t used to seeing addition, subtraction, or multiplication facts written or spoken in that backwards way. Coleen Young made an excellent slide presentation titled good-mathematicians-can-go-backwards. We ALL can go forward in mathematics so much more confidently once we learn to go backwards.

Coleen recommends many resources including the Find The Factors puzzles. Each of these puzzles can be solved and then completely filled out as a multiplication table if you will go backwards first and find the factors of the clues in the grid. Think what two numbers multiplied together give you 49? Give it a try!

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12 thoughts on “434 Going Backwards Is Important!”

I think either of your clues gave too much in the way of a clue. 71 immediately tells me to look at primes in the …, 67, 71, 73, … range, and telling me six primes are involved again points me towards a prime close to 434÷6 being in the middle of the six. But if all I know is that a group of successive primes is involved I’ve got to work a bit harder (so please set us one at the first opportunity!)

I think so. Otherwise the clues are just too helpful. Maybe the puzzles won’t be all that interesting anyway, but without any clues at all I won’t necessarily know if I’m looking for an odd or even number of primes.

Try it and let’s find out! As you point out, if 2 is not in the picture I can immediately eliminate 50% of the possibilities I need to look at, so that’s one reason for limiting the amount of information you should give.

I sometimes joke that to completely understand a math concept, you should be able to do it forward, backward, sideways or from any direction 🙂

Since math is perceived to be a rigid subject, many assumed that you can’t use different methods in solving a problem but of course, that’s not the case. There are problems especially in number theory and combinatorics that can be solved in several ways from several directions after all.

Thank you. Too much dependence on calculators does make sense. I’ve seen 14 year olds who couldn’t divide 18 by 2 without one. I haven’t seen very many 8 or 9 years old using them, however. I think never seeing or hearing backward equations like 72=8×9 makes a difference, too.